The present invention concerns a spinal instrumentation system utilizing elongated members extending along the length of the spine and attached to multiple vertebrae by fixation elements, such as bone screws. In particular, the invention concerns anterior instrumentation, together with a surgical technique for implanting the instrumentation. The invention also contemplates a surgical revision technique for this spinal instrumentation.
Historically, correction of spinal disorders and treatment of spinal injuries was approached posteriorly, or namely from the back of the patient. Initially, the anterior approach to spinal instrumentation, that is from the front and side of the patient, was not favored, due to the unfamiliarity of this approach to spinal surgeons and due to the fear of severe complications, such as neurovascular injury or compromise of the spinal cord. However, in the face of some reported difficulties in addressing correction of thoracolumbar and lumbar scoliotic curvatures from a posterior approach, surgeons sought anterior forms of correction and stabilization. One such technique was developed by Dwyer in Australia during the 1960's in which a staple-screw construct was applied to the convex side of the scoliotic curvature. The screws were connected by a cable and correction was obtained by applying compressive forces at each instrumented level. The anterior spinal compression produced tensile forces within the cable which in turn generated a corrective bending moment at each of the vertebral levels.
On the heels of Dwyer's success, other anterior instrumentation followed. Further, surgeons began to recognize that certain spinal treatments were best approached anteriorly, rather than posteriorly. Anterior approaches give direct access to the intervertebral disc space for anterior release and interbody fusion. Presently, common indications for anterior instrumentation include: lumbar scoliosis with deficient posterior elements; thoracolumbar curves with extreme lordosis; paralytic thoracolumbar scoliosis requiring both an anterior and a posterior fusion; thoracolumbar spine trauma, such as burst fractures; and degenerative conditions of the vertebral body. In the case of burst fractures, it is known that neurocompression occurs from the anterior direction. Further, anterior debridement of fracture fragments is frequently believed to be a more effective means to decompress the spinal canal as opposed to known posterior techniques.
Since the initial Dwyer instrumentation, many anterior plate and rod systems have been developed, such as the systems of Dunn, Kostuik-Harrington, Zielke and Kaneda. Many of these systems permit dynamic distraction of the vertebrae followed by direct compression of implanted bone graft contained within the resected disc space and after decompression of the neural elements.
Many of these anterior systems can lead to complications. Some of the more prominent problems that have occurred involve failure of the fixation components, and an often high incidence of loss of reduction or correction. Many of the difficulties in this respect can be traced to the vertebrae instrumented at the end of the construct where the loads on the instrumentation are the greatest. In some cases, bicortical purchase of vertebral body screws has been found to assure a more solid fixation at the ends of the construct and to protect against dislodgement of the screws. There does, however, still remain a need for an anterior instrumentation that can provide adequate correction of spinal deformities and that can be easily implanted. In addition, the system must ensure a strong fixation that will not deteriorate over time resulting in a loss of correction.
In some cases, it has been found that revision surgery is necessary, even when following the best possible surgical implantation of the instrumentation. Frequent indications for revision of spinal instrumentation include extension of existing instrumentation, and replacement of failed implants. In the cases involving early spinal implants, revision required cutting away the spinal implants. As implant design became more sophisticated, capabilities were developed for revision surgery that was relatively safe to the patient and non-destructive to implants, particularly those implants that were intended to be retained.
In systems using bone screws, revision surgeries can significantly compromise the vertebral body. In addition, in certain anterior approaches where stronger fixation is essential, revision procedures to replace failed components may necessarily compromise the new construct.
In view of these difficulties, there is a need for a spinal fixation system that is readily suited for revision surgery. Specifically, the system must be suitable for the addition or removal of components by way of revision without sacrificing either an existing construct or eliminating the possibility of implanting a new, more stable construct. In addition, there is a need for revision techniques that permit complete removal of a construct once fusion has occurred, again without compromising the spine or the implants.